#include<bits/stdc++.h>

using namespace std;

  

// User defined stack that uses a queue

class Stack

{

    queue<int>q;

public:

    void push(int val);

    void pop();

    int top();

    bool empty();

};

  

// Push operation

void Stack::push(int val)

{

    //  Get previous size of queue

    int s = q.size();

  

    // Push current element

    q.push(val);

  

    // Pop (or Dequeue) all previous

    // elements and put them after current

    // element

    for (int i=0; i<s; i++)

    {

        // this will add front element into

        // rear of queue

        q.push(q.front());

  

        // this will delete front element

        q.pop();

    }

}

  

// Removes the top element

void Stack::pop()

{

    if (q.empty())

        cout << "No elements\n";

    else

        q.pop();

}

  

// Returns top of stack

int  Stack::top()

{

    return (q.empty())? -1 : q.front();

}

  

// Returns true if Stack is empty else false

bool Stack::empty()

{

    return (q.empty());

}

  

// Driver code

int main()

{

    Stack s;

    s.push(10);

    s.push(20);

    cout << s.top() << endl;

    s.pop();

    s.push(30);

    s.pop();

    cout << s.top() << endl;

    return 0;

}